The present invention relates generally to projection optical systems and exposure apparatuses for use with lithography, and more particularly to a catoptric or reflection type projection optical system, an exposure apparatus, and a device fabricating method, which use ultrarviolet (“UV”) and extreme ultraviolet (“EUV”) light to project and expose an object such as a single crystal substrate for a semiconductor wafer, and a glass plate for a liquid crystal display (“LCD”).
Along with recent demands on smaller and lower profile electronic devices, finer semiconductor devices to be mounted onto these electronic devices have been increasingly demanded. For example, a design rule for a mask pattern requires that an image with a size of a line and space (“L & S”) of less than 0.1 μm be extensively formed, and predictably, it will further move to a formation of circuit patterns of less than 80 nm in the future. L & S denotes an image projected to a wafer in exposure with equal line and space widths, and serves as an index of exposure resolution.
A projection exposure apparatus, which is a typical exposure apparatus for fabricating semiconductor devices, includes a projection optical system that projects and exposes a pattern drawn on a mask (reticle) onto a wafer. Resolution R of a projection exposure apparatus (a minimum size which enables a precise transfer of an image) can be given by using a light-source wavelength λ and the numerical aperture (NA) of the projection optical system as in the following equation:                     R        =                              k            1                    ×                      λ            NA                                              (        1        )            
As the shorter the wavelength becomes and the higher the NA increases, the better the resolution becomes. The recent trend has required that the resolution be a smaller value; however it is difficult to meet this requirement using only the increased NA, and the improved resolution expects use of a shortened wavelength. Exposure light sources have currently been in transition from KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm) to F2 excimer laser (with a wavelength of approximately 157 nm). Practical use of the EUV light is being promoted as a light source.
As a shorter wavelength of light limits usable glass materials for transmitting the light, it is advantageous for the projection optical system to use reflection elements, i.e., mirrors instead of using many refraction elements, i.e., lenses. No applicable glass materials have been proposed for the EUV light as exposure light, and a projection optical system could not include any lenses. It has thus been proposed to form a catoptic reduction projection optical system only with mirrors.
A mirror in a catoptric reduction projection optical system forms a multilayer film to enhance reflected light and increase reflectance, but the smaller number of mirrors is desirable to increase reflectance of the entire optical system. In addition, the projection optical system preferably uses the even number of mirrors to avoid mechanical interference between the mask and the wafer by arranging the mask and the wafer a: opposite sides with respect to a pupil.
In addition, as the critical dimension (or resolution) required for EUV exposure apparatuses is smaller than a conventional value, higher NA is needed, such as NA=0.2 for a wavelength of 13.5 nm, although conventional three- or four-mirror systems have a difficulty in decreasing wave front aberration. Accordingly, for increased degree of freedom of corrections to wave front aberration, it has been necessary to make a mirror aspheric with the number of mirrors around six (which may be referred to as “six-mirror system” in this application”); many six-mirror systems of this type have been proposed (as seen in Japanese Patent Publications Nos. 2000-100694 and 2000-235144.
Usually, an exposure apparatus locates a mask as an original form of a pattern at an object plane. As this mask should be exchanged and scanned in exposing a pattern, a stage mechanism should be located near the mask at sufficiently wide space when the above six-mirror system is to be applied to an actual exposure apparatus.
As the exposure apparatus is usually accommodated in a clean room, and its entire size is limited due to facility restrictions and thus the span of the optical system is limited. In exposure using the EUV light, it is absorbed in the air and the optical path should be made vacuum. Therefore, the size of the optical system is limited from vacuum drawing efficiency. Thus, there should be a sufficient interval between the object plane and (a reflective surface of) a mirror closest to the object surface without increasing the span of the optical system (a distance from the object plane to the image plane) and the effective diameter.
While the catoptric projection optical system in Japanese Patent Publication No. 2000-100694 discloses two embodiments using six-mirror systems with NA of 0.14 and NA of 0.16, the first embodiment with NA of 0.14 is substantially a five-mirror system because the fourth mirror M4 is a plane mirror, which has a difficulty in increasing NA. In addition, the second embodiment with NA of 0.16 uses a spherical mirror for the fourth mirror M4, increasing the degree of design freedom, but requires a distance from the object plane to the image plane is 2 m or greater and has a difficulty in realization. In addition, the maximum effective diameter of the mirror is about 450 mm, and becomes larger as NA becomes higher.
Either embodiment forms an intermediate image between the second and third mirrors M2 and M3, and arranges four mirrors from the intermediate image to the image plane. Therefore, as a beam width becomes larger with higher NA, a beam enlarges particularly from the intermediate image to the image plane, and has a difficulty in separating mirrors from a beam other than a desired beam and arranging them. Therefore, neither the first embodiment nor the second embodiment can achieve high NA of 0.16 or greater. A compulsory attempt to arrange mirrors would cause another problem to make the maximum effective diameter larger.
Moreover, a distance between the object plane and the mirror M2 closest to the object plane is so small as 20 mm to 30 mm. For example, as shown in FIG. 2, a distance between the second mirror M2 and the mask R is very long. It would be understood from this that it is difficult to apply two optical systems disclosed in Japanese Patent Publication No. 2000-100694 to an actual exposure apparatus.
On the other hand, Japanese Patent Publication No. 2000-235144 also discloses catoptric projection optical systems as six-mirror embodiments with high NAs of 0.2, 0.28 and 0.30. Similarly, however, as a distance between the object plane and the mirror M2 closest to the object plane is so small as 80 mm to 85 mm, it is difficult to arrange a stage mechanism for scanning a mask located on the object plane. In addition, it is the fourth mirror M4 that has the maximum effective diameter in either embodiment, and the diameter is so large as 540 mm or greater for NA of 0.2. The largest effective diameter is a diameter larger than 650 mm for NA of 0.28, and the mirror's maximum effective diameter increases simultaneous with high NA.